/Engine/Source/Runtime/Core/Public/Containers/UnrealString.h
Append a single character and return a reference to this
template<typename AppendedCharType, std::enable_if_t< [TIsCharType](API\Runtime\Core\Traits\TIsCharType)< AppendedCharType >::Value > *> [FString](API\Runtime\Core\Containers\FString) & operator+= ( AppendedCharType Char )